1. Field of the Invention
The present invention relates generally to electrochemical sensors.
Particularly, this invention relates to an electrochemical sensor having a prolonged useful life and/or other improved performance characteristics. More particularly, this invention relates to a polymeric membrane ion-selective electrode (ISE) having a prolonged useful life and/or other improved performance characteristics. Even more particularly, this invention relates to an ISE polymeric membrane with a gelled membrane composite boundary between the sample being tested and the polymeric membrane.
2. Description of the Prior Art
The conventional detection of various chemical species customarily uses electrochemical sensors specially designed for determining a particular chemical species' concentration, more accurately described as its activity, in a solution. The determination is based on the fact that within certain limits, the potential of the electrode is directly proportional to the logarithm of the chemical species' activity. These electrochemical sensors generally have sensing membranes specifically formulated for measuring the chemical species of interest. One of the essential properties of an electrochemical sensor is its selectivity. In other words, the results that are obtained are maximally independent of other chemical species/ions which are present in the sample being tested.
Selective membrane sensors originated in the basic pH glass membrane electrode. The advent of crystal and liquid membrane sensors led to the development of ion-selective electrodes. Combining these sensors with microporous synthetic membranes resulted in electrochemical sensors for measuring carbon and sulfur dioxide, ammonia, hydrogen sulfide, and other dissolved gases in blood urine and body fluid samples. Coupling biological reagents to gas sensors and ion-selective electrodes resulted in biosensor systems involving enzymes, bacteria, tissue cells, and immuno-agents. Other types of sensors have also been developed which include affinity, enzyme-linked immunoadsorbent, immune-complex, antigen, and antibody sensors. The ion-selective electrode is the key element of many biosensors.
ISEs have many applications in the fields of medicine, engineering, industrial processing control, education, and research. They are especially useful in clinical and environmental chemistry where large numbers of samples are processed.
Basically, the sensing membrane is made of glass or various polymers such as polyvinyl chloride (PVC), silicone and the like. Sensors using glass membranes such as sensors for pH and sodium are relatively resistant to leaching under normal test conditions. Even when used in media such as human serum containing lipophilic agents, a glass-membrane type sensor can usually be cleaned and reconditioned thus extending the sensor's useful life. A polymeric-membrane type ISE, on the other hand, is more sensitive to the conditions that cause failure, thus having a much shorter useful life as compared to glass-membrane type sensors. One of these conditions is the presence of interfering ions or substances.
Various attempts have been made in the past to couple multiple sensors together and to prolong sensor life.
U.S. Pat. No. 4,568,445 (1986, Cates et al.) discloses an electrode system for an electrochemical sensor for measuring vapor concentration having a plurality of electrically conductive sensing segments isolated from each other. The sensing segments are unencapsulated and are each coated with respective ones of ion conducting sensing materials to characterize each of the segments except one which is used as a reference electrode. The outer surface area of the sensing materials and the reference electrode is coated with a layer of nonaqueous electrolyte which serves as a sorption/desorption medium. The electrolyte is covered by a semipermeable thin film membrane made of silicone.
U.S. Pat. No. 4,851,088 (1989, P. Chandrasekhar et al.) discloses an electrochemical system for the detection of carbon dioxide which includes a single cell chamber exposed to the sample medium through a polymeric barrier membrane, a single set of electrodes and utilizes an aprotic nonaqueous, gelled solvent/electrolyte medium which allows measurement of Carbon Dioxide in the presence of both oxygen and water vapor.
U.S. Pat. No. 5,132,345 (1992, S. J. Harris et al.) discloses an ion-selective polymeric membrane for an electrochemical sensor for use in analytical chemistry. The polymeric membrane is made of a supporting matrix, usually PVC, and an ionophore selected from calixarene or oxacalixarene derivatives.
U.S. Pat. No. 5,326,449 (1994, David D. Cunningham) discloses a sensor device for measuring the concentration of an analyte in solution. The device includes a composite membrane which incorporates a porous membrane containing an immobilized biologically-active protein and at least one other membrane. The other membrane may optionally be a blocking membrane, partly embedded in the porous membrane, which is adapted to exclude low molecular weight interfering species such as ascorbic acid, or it may be a protecting membrane which is useful for preventing high molecular species from fouling the porous membrane. Both a blocking membrane and a protecting membrane may be included in the composite membrane simultaneously.
U.S. Pat. No. 5,567,290 (1996, P. M. Vadgama et al.) discloses sensor devices for examining fluid samples having, between the sample under examination and a detector, a membrane made of polyvinyl chloride in unplasticized form. This membrane material acts as a barrier to paracetamol and sugars but is permeable to hydrogen peroxide and to oxalate.
Therefore, what is needed is an ISE that has a longer uselife and better performance than ISEs currently available. The uselife of an ISE is defined as the length of time an ISE continues to function properly and reliably for its intended use. What is further needed is an ISE that is more easily cleaned during normal operation inhibiting the deposit of proteins and lipid compounds. What is still further needed is a polymeric membrane sensor that allows the passage of the chemical species to be measured while inhibiting the extraction of ionophores and plasticizers out of the polymeric membrane matrix and surfactant-type agents into the membrane matrix.